Corrosion Monitoring of Reinforced Concrete Structures
A proper test plan should take into account the following considerations:
- Exposure condition (Access to moisture, oxygen, exposed to de-icing salts, other chemicals, carbon dioxide);
- Geometry of the structures and components;
- Access to structural and non-structural components;
- Dimension and scale of the area under investigation;
- Importance of the element, etc.
The following inspection procedures are widely used during a routine corrosion monitoring program:
Chain Drag Delamination Survey
Chain dragging is widely used to detect delamination in concrete bridge decks. The concept behind this very simple method is the unique hollow sound that is made by dragging chain across the delaminated surface. The test is used to identify potentially delaminated areas on the deck slab.
The practice is difficult when vehicles are moving over the deck, and distinguishing the hollow sound becomes difficult.
The main disadvantage of the chain dragging method is that they can only provide results if the defect and deterioration are in well advanced stages. Early diagnosis of damage mechanism is not possible using this technique. Application of the method is somewhat difficult during heavy traffic.
Measuring the chloride content in concrete cover (and at the rebar surface) is a classic method in corrosion investigation of concrete structures. Powder samples are obtained from different depths (usually 5 depths from exposed surface to rebar level), and chloride profile is developed. Diffusion coefficient can be determined in this method. The main advantage is to understand if the concentration of chloride ion is higher than the threshold value (the amount required for depassivating the protective film over rebar).
Electrical Resistivity Measurement
The measurement of electrical resistivity of concrete provides useful information about the microstructure of concrete material. The test procedure has since been standardized by AASHTO TP 95.
It can provide useful information about the resistance of concrete material to penetration of chloride ion. Electrical resistivity measurements are effected by moisture content, salt content of concrete, and presence of steel rebar. This makes the task of data interpretation difficult.
Half-Cell Corrosion Potential Mapping
The test helps in the quality assurance of concrete repair and rehabilitation. Several standard associations have standardized the test procedure including the ASTM C 876, UNI 10174 and RILEM TC 154. Depending on the measured half cell corrosion potential value, the probability of active corrosion is determined.
Ground Penetrating Radar – GPR
GPR is mainly used for sub-surface imaging of concrete decks. However, with the recent developments in the radar technology, the technique is now being used for corrosion monitoring as well. The ease of use, and the fact that scanning can be performed at the highway speed makes this method particularly important for large bridge decks. However, using GPR for corrosion monitoring needs trained and experienced individuals.
GPR uses electromagnetic radiation in microwave band of the radio spectrum, emitting radar pulse into the medium and detecting the reflected signals from subsurface media. A superposition of reflected signals is used to image the subsurface area. GPR is more accurate and robust to monitor the post-corrosion-initiation signs and side effects. It is obvious that GPR is not effective to monitor the corrosion signs at early stages.
Source: FPrimeC Solutions